At present, a main method for detecting whether there is a water drop on a touch screen is alternate coding detection, the principle of which is as follows. All the channels of the touch screen are uniformly numbered and the number starts with 0. For example, a driving signal is applied to an even channel, while an odd channel is set to be floating (i.e. no driving signal is applied to the odd channel). When there is no water drop or hand touch, one frame of data is sampled and serve as reference values. When only water drop exists, a sample value detected in the even channel gets smaller because partial signals are coupled through the water drop to an odd channel adjacent to the even channel. Therefore, a difference value (i.e., a value obtained by subtracting the detection sample value from the reference value) of the even channel is a positive value. At this time, a sample value detected in the odd channel becomes larger due to coupled signals from the even channel, and then a difference value of the odd channel is a negative value.
In setting the reference values, the odd channel is set to be floating, driving signal is applied to the even channel (i.e. the coding channel), and a direct current (DC) voltage is input to each non-inverting input end of a differential programmable gain amplifier (PGA) corresponding to each channel. At this time, one frame of data is sampled from each channel through an inverting input end of each PGA, and the frame of data is processed to be as the reference values. That is, in setting the reference values, a DC component is subtracted from a driving signal via hardware by inputting a DC voltage to the non-inverting input end of the PGA.